Chemical purification and modification of cellulose fiber



Patented Apr. I 7, 1936 PATENT OFFICE CHEMICAL PURIFICATION AND MODIFI- CATION F CELLULOSE FIBER George A. Richter, Berlin, N.

Brown Company, Berlin,

of Maine IL, assignor to N. 11., a corporation No Drawing. Application January 10, 1935, Serial No. 1,267

15 Claims.

This invention relates to the chemical purification and modification of cellulose fiber of the nature of chemically preliberated wood pulp and cotton, more particularly for the purpose of enhancing its value for conversion into cellulose esters, derivatives.

The ordinary forms of cellulose, such as cotton or chemical wood pulp, used for making cellulose derivatives can generally be significantly improved for such purpose. Thus, it is frequently desired that the derivatives be possessed of low solution viscosity; and this quality is lacking in derivatives prepared from ordinary chemical wood pulps or cotton. Again, derivatives prepared from ordinary chemically preliberated pulps, such as sulphite or kraft wood pulp, are of inferior purity and physical characteristics on account of the high residue of non-alpha cellulose components therein.

In accordance with the present invention various kinds of cellulose pulps or fibers are improved as raw material for conversion into all sorts of cellulose derivatives by a process inclusive of at least three steps, namely, initial and tertiary steps of treating the pulp with a merlcerizing liquor and a secondary step of hydrolyzing the initially mercerized pulp. I have found that such a combination of steps is advantageous not only in that the solution viscosity of the pulp or fiber is markedly reduced but further in that such reduction in solution viscosity is attained while producing a product of high alpha cellulose content or purity. More specifically, the initial treatment with mercerizing liquor serves to extract impurities from the pulp, to reduce its solution viscosity, and to transform the pulp fibers to a state such that they are highly amenable to hydrolyzing action and attendant marked reduction in solution viscosity. The final treatment with mercerizing liquor not only extracts the impurities or degraded celluloses developed in the pulp on account of its hydrolysis but does so while preserving the low solution viscosity at tained in the pulp. It might be observed that while it is possible to use weak alkaline liquors to dissolve some non-alpha cellulose impurities occurring in cellulose pulp, such liquors are not nearly as effective as a mercerizing liquor in extracting pentosans from the pulp; and their application at room or only moderately elevated temperature to hydrolyzed pulp without the use of a mercerizing liquor results in sacrifice of considerable of the low solution viscosity value of the pulp. In other words, weak alkaline liquors cellulose ethers, and other cellulose alone, unless used under especially high temperature, tend to undo considerable of the solution viscosity-lowering effect on the pulp realized by hydrolyzing the pulp.

Because kraft wood pulp represents a fiber 5 comparatively resistant to purification on account of its high pentosan content, I shall describe how such pulp can be advantageously modified by the practice of the present invention. Ordinary kraft wood pulp has extremely high solution visl0 cosity and its content of non-alpha cellulose components, especially pentosans, is quite substantial. By applying a mercerizing liquor to such pulp, it is possible to reduce its pentosan content markedly. For instance, kraft pulp having a pentosan content of about 9% can thus be brought to a pentosan of only about 5%. However, for many purposes such residual pentosan content is still excessive. When, however, the pulp is suitably hydrolyzed and then again subjected to the action of amercerizing liquor, it is found that the pentosan content of the pulp can be reduced to as low as about 0.5% to 1.0%. The pentosan content of other kinds of pulp can be reduced by similar processing to even lower values. For instance, ordinary sulphite wood pulp, which initially has a distinctly lower pentosan content than kraft pulp, can be purified by the practice of the present invention to a pentosan content of as low as 0.2% to 0.3%. The purified cellulose products prepared in accordance with the present invention are not only of high alpha cellulose content, say, about 94% to 98%, and very low pentosan content, but are of low solution viscosity, say, about 0.05 to 1.0. Such purified cellulose products are of great value in making cellulose esters, cellulose ethers, and other cellulose derivatives. Thus, they may be converted advantageously into such cellulose derivatives as cellulose nitrate, cellulose acetate, cellulose xanthate, alkyl ethers, hydroxy alkyl ethers, cuprammonium cellulose etc.

In carrying out the primary and tertiarwti'eatments of the pulp with mercerizing liquor, various conditions of temperature, time, and alkalinity may be employed. Thus, the treatment may be performed at room temperature, say, 20 C., with caustic soda solution of about 18% strength for about to 2 hours or longer. Of course, stronger solutions may be used. On the other hand, solutions of as low as about 10% strength may be advantageously applied to the pulp. For instance, a 12% caustic soda solution at, say, 50 C. is quite effective as a purifying reagent when used in the tertiary treatment. A

higher than normal temperature in the tertiary treatment is advantageous when the refined pulp is to be acetylated, as under such temperature conditions it is possible to prcduce purified fiber of higher activity toward the acetylating reagent than when normal cjr sub-normal temperature conditions are maintained during the tertiary treatment. when, however, the purified product is to be converted into the nitrate, xanthate, and other cellulose derivatives, it may be preferable to carry out the tertiary as well as the primary treatments at sub-normal temperature conditions, say, at 10 C. or even iower. Each of the three treatments of the pulp may be performed at a stock density er consistency of about 5% to the fiber suspension being stirred or agitated'so to ensure a substantially uniform treatment or the fiber. The stock is washed after each treatment, washing after the primary and tertiary treatments preferably being done in a counter-current washer so as to make possible a recovery of alkaline liquor with minimum dilution for reuse in the treatment of other pulp.

The intermediate or hydrolyzing treatment can be accomplished in various ways. Thus, the pulp may he digested at elevated temperature, say, at 80 to 150? C. in weak solutions of mineral acids, such as 0.1%- to 0.5% solutions of hydrcchloric, sulphuric, or nitric acids. Or concentrated mineral acid solutions may be applied to the pulp at room temperature, for instance, a 20% sulphuric acid solution at 20 C. Acid salts, such as sodium acid sulphate or sulphite may be used in lieu of dilute mineral acid solutions. In-

deed, water alone'cr solutions of neutral or weakly basic compounds, such as sodium sulphite, so-, dium borate, and 'sodiurn phosphate, may serve as the hydrolyzing reagents, provided that the pnlp is digested therein at sufiiciently elevated temperature, say, at temperatures' of about 300 to 400 F. Chlorine, sulphur dioxide, or other acid gases or vapors maybe used in either gaseous or dissolved state as the hydrolyzing reagents. In any event, treatment with the hydrolyzing reagent carried on for a sufiicient period of time to cause the solution viscosity of the initially mercerizedpulp to fallmarkedly, e., to only a small fraction of the solution viscosity of the original pulp, while preferably preserving substantially unimpaired the fibrous structure of the pulp. In other words, it is preferable that r the hydrolyzinglftreatme'nt be performed under such conditions c-f temperature, of time, and concentration of hydrolyzing reagent as not to transform the pulp fibers to a pulverulent or fiberless; state, since such latter state in the cellulose gives; iise to excessive 'loss of cellulose during the subsequent or tertiary treatment and makesifor difficulty in recovering the cellulose from suspension.

The finished or purified product produced pursuant to the practice of the present invention is possessed of? great softness and absorbency. It may thus be formed on paper-making ma;- chines into soft or flufiy sheets or papers, which, besides lending themselves 'to use as ;raw materlal for conversion into cellulose derivatives,

can serve to advantage as bases for a wide variety aoaaeoe tion or individualization of the fiber units and thus to yield a voluminous 9r bulky mass of cellulose fibers that lends itself to rapid and substantially uniform ponversion into cellulose derivatives. .By virtue of its purity, the purified cellulose woduct of the present invention may be used as such or after reductiomto a pulverulent state, as in a hammer or ball mill,- as a filler in papers, fabrics, molded plastic products, and others.

Other specific examples of procedure embodying the present invention .will now be given.

Example A.-.-Refined wood pulp of an alpha cellulose content cf about 94% was treated for about one hour with an 18% caustic soda solution at 20? C. The resulting mercerized pulp was was used instead of dilute hydrochloric acid solution in the second or hydrolyzing treatment of ExampleflA whilerepeating the other treatments of Example A, the resulting prodntt had an alpha cellulose content of 95%., a pentosan content of 0.9%, and a solution viscosity of 0.35.

Example C.-]Vhen caustic soda solution of 12% strength was used at 60 C. in the first and third treatments of Example A while repeating the other treatments of Example A, the resulting -product had analpha cellulose content of 96%,

la pentosan content of 1.0%, and a solution of viscosity of 1.0. Example D.When the second or hydrolyzing treatment of Example A was performed for two hours at 175 C. with a 0.2% solution of sodium sulphite while repeating the other treatments of Example A, the resulting productyhad an alpha cellulose content of 96 a pentosan content or i 1.0%, and a solution viscosity of 0.6. I J 5 Example E.--In this example, kraft pulp W of a pentosan content of about 8% and having very high solution viscosity was used as raw material. The pulp was treated for about one hour with a 10% caustic soda solution at C., washed,

then hydrolyzed for about one hour with 0.1 normal sulphuric acid solution at 100 C., again washed, treated for about one hour with 18% caustic soda solution at 20 C., and again washed. The resulting product had an alpha cellulose content of 95.2%, a pentosan content 010.9%, and a solution viscosity of 0.4.,

Example F.=--This example was similar to Example E excepting that a 15% caustic soda solution at 20 C. was used in the first treatment and 0.2 normal sulphuric acid solution was used in the second or hydrolyzing treatment. The resulting product had an alpha cellulose content of 96%, a pentosan content of 0.6%, and a solution viscosity of 0.35.

In any of the preceding examples; it may be desirable to add oxidants, such as hvpochlorites, permanganates, bicl romates; and peroxides, to the mercerizing liquor, especially the first mercerizing liquor to promote the reduction of the solution visefosity of the cellulose pulp or fiber.v

In some instances, however, it may be desirable to add reducing agents rather than oxidmng agents to the mercerizing liquors, for instance, such reducing agents as sodium sulphide and/or sodium sulphite. The hydrolyzing liquor may, if desired, contain an oxidant which serves to promote the reduction of the solution viscosity of the cellulose pulp or fiber. Oxidants, such as hypochlorites. permanganates, and bichromates, may be added to such hydrolyzing reagents as solutions of mineral acids or acid salts. Instead of subjecting the hydrolyzed pulp or fiber directly to the action of strong or mercerizing alkaline liquor, it may be desirable first to treat the hydrolyzed pulp or fiber with weakly alkaline liquor. Thus, such weakly alkaline and comparatively inexpensive liquors as solutions of lime or solutions of sodium carbonate may be used at elevated temperature, ray, 80 to 100 C., to extract from the hydrolyzed pulp or fiber those degraded celluloses that dissolve readily, thereby minimizing the work of purification to be done by the final mercerizing liquor and thus minimizing the consumption of the more expensive caustic soda used in the final mercerizing liquor.

As already indicated, the process of the present invention may be applied to cellulose pulps or fibers of widely variant characteristics, such as raw or unbleached chemical wood pulps, bleached chemical wood pulps, such as bleached sulphite pulps, refined wood pulps of bleached and unbleached character, cotton fiber, cotton and linen rag stocks, etc. The process may be applied to the pulp in loose or bulk form, in which case the fibers are suspended or digested in the various treating reagents; or it may be performed with the pulp or fiber in interfelted or sheet form, in

which latter case, the sheets may be steeped as a batch in baths of the various reagents or one or more sheets may be progressively run through the various reagents.

In using the expression "mercerizing liquor in the foregoing description and in the appended claims, I mean a strong solution of caustic soda or equivalent alkali, and more particularly a caustic soda solution of at least about 10% strength and applied at temperatures ranging from about to 80 C. It is genera ly preferable to apply the solution to the fiber at a temperature not exceeding room temperature, say, 20 0., especially when the solution is of only about strength, but stronger solutions, for instance, caustic soda solutions of about and greater strength may be applied with good effectiveness at temperatures up to about 80 C. and are advantageous especially when the resulting purified product is to be acetylated. It might be observed that after the initial mercerizing treatment, the fiber may be centrifuged, or thickened, or pressed, or otherwise treated for the removal of excess alkaline liquor, whereupon the resulting alkali-cellulose may be aged, as in a shredded condition, for a considerable period of time, say, for twenty-four hours or longer, to acquire lower solution viscosity by virtue of exposure to the air. The aged fiber may then be washed and put through the other treatments of the present invention.

The solution viscosity values hereinbefore given were determined by converting the fiber into a cuprammonium cellulose solution of prescribed cellulose concentration and then measuring the time of efilux of such solution under standard conditions throughan orifice of standard size. More specifically, the solution viscositieshereinbefore given are in absolute C. G. S. units or poises and were determined by measuring the viscosity of a solution of six grams of cellulose fiber in a cuprammonium cellulose solution composed of 225 cc. of 28% ammonia water containing 9 grams of so-called "copper hydrate powder.

This copper hydrate powder is in reality basic copper nitrate corresponding in composition to the formula Cu(NO3)2.3Cl1(OI-I)2. The C. G. S. unit is employed because it is definite, denoting a viscosity 100 times that of water at 20 C., wherefore, a cuprammonium cellulose solution of standard composition identifying a fiber as having a solution viscosity of 10 is 1000 times as vis- 0 .us as water at 20 C. The method of determining or measuring solution viscosity of cellulose fiber used herein is that described by me in much greater detail in Industrial and Engineering Chemistry, volume 23, page 136, 1931; and inasmuch as the description of my viscosity-testing method as given in that publication afiords the particular criten'cn or test used herein, it is to be understood that my reference to such description is intended to incorporate such description as a part hereof.

I claim:-

1. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber under conditions to efiect a lowering of its solution viscosity to a value constituting only a small fraction of that of the original fiber, and purifying such hydrolyzed fiber in a mercerizing liquor.

2. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise pm'ify- 1 ing the fiber in a mercerizing liquor, then hydrolyzing the fiber under conditions to eifect a lowering of its solution viscosity to a value consiituting only a small fraction of that of the original fiber while largely preserving its fibrous structure, and purifying such hydrolyzed fiber in a mercerizing liquor.

3. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature cf chemically preliberated wood pulp and c tton, these steps which comprise purifying the fiber in a caustic soda solution of at least about 10% strength, then hydrolyzing the fiber under conditions to effect a lowering of its solution viscos'iy to a value constituting only a small raction of that of the original fiber, and purifying such hydrolyzed fiber in a caustic soda solution of at least about 10% strength.

4. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chcmicaliy preliberated wood pulp and c -t'on. those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a suflicient period of time to effect a marked lowering of its solution viscosity, and purifying such hydrolyzed fiber in a mercerizing liquor.

5. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufficient period of time to efiect a marked lowering of its solution viscosity while largely preserving its fibrous structure, and purifying such hydrolyzed fiber in a mercerizing liquor.

6. In the production of purified cellulose fiber 4- -aosaooe of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in" a caustic soda-solution of at least about 10% strength, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufiicient period of time to efiect a marked lowering of its solution viscosity while largely preserving its fibrous structure, and purifying such hydrolyzed fiber in a caustic soda solution of at least about 10% strength 7. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufflcient period of time to effect a marked lowering of its solution. viscosity, and purifying such bydrolyzed fiber first in a weakly alkaline liquor and then in a strongly alkaline liquor.

8. In. the production of purified cellulose fiber of low solutionviscosity from cellulose fiber of the nature of chemically preliberated wood'pulp and cotton, those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufilcient pe- Y riod of time to eflz'ect a marked lowering of its solution viscosity, and purifying such hydrolyzed fiber in a weakly alkaline liquor and then in a mercerizing liquor.

9. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in a mercerizing liquor, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufiicient period of time to effect a marked lowering of its solution viscosity while largely preserving its fibrous structure, and purifying such hydrolyzed fiber first in a weakly alkaline liquor and then in a strongly alkaline liquor.

10. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of chemically preliberated wood pulp and cotton, those steps which comprise purifying the fiber in an alkaline liquor of at least about 10% causticity, then hydrolyzing the fiber in a dilute solution of mineral acid at elevated temperature and for a sufilcient period of time to effect a marked lowering of its solution viscosity while largely preserving its fibrous structure, and purifying such hydrolyzed fiber first in a weakly alkaline liquor and then in an alkaline liquor of at least about 10% causticity.

the washed fiber in a dilute solution of mineral acid at elevated temperature and for a sufilcient period of time to efiect a marked lowering of its solution viscosity, and subjecting the hydrolyzed fiber to the purifying action of a mercerizing liquor. I 7

12. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber oi the nature of preliberated wood pulp and cotton, those steps which comprise initially subjecting the fiber to the purifying action of a mercerizing liquor, then hydrolyzing the fiber in a solution of mineral acid selected from a group of hydrochloric, sulphuric and nitric acids, and subjecting the hydrolyzed fiber to the purifying action of a mercerizing liquor.

13; In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of preliberated wood pulp and cotton, those steps which comprise initially subjecting the fiber to the purifying actionof a mercerizing liquor, then hydrolyzingsthe fiber in an 0.1% to 0.5% solution of a mineral acid selected from a group consisting of hydrochloric, sulphuric, and nitric acids, and subjecting the hydrolyzed liquor.

14. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of preliberated wood pulp and cotton, those steps which comprise initially subject' ing the fiber to the purifying action of a mercerizing liquor, then hydrolyzing the fiber at elevated temperature in a dilute solution of mineral acid selected from a group consisting of hydrochloric,

sulphuric and nitric acids, and subjecting the hydrolyzed fiber to the purifying action of a mercerizing liquor.

15. In the production of purified cellulose fiber of low solution viscosity from cellulose fiber of the nature of preliberated wood pulp and cot tori, those steps which comprise initially subyfiber to the purifying action of a mercerizing jecting the fiber to.the purifying action of a mer- GEORGE A. RICHTER. 

